1. Field of the Invention
The invention relates to integrated circuits and more specially to integrated circuits in which there is provision for antifuses which can be used to create connections after the manufacture of the integrated circuit.
By way of an example, to understand what these antifuses are, it may be recalled that it is now common practice, in integrated circuits designed for chip cards, to set apart non-volatile memory zones to which the user should be denied access. These memories are therefore filled with information elements, after which an antifuse is blown to insulate the prohibited memory zone so that its contents can no longer be transmitted to the external terminals of the card or so that the memory can no longer be written in from these terminals. Another application of the antifuses is the making of read-only memories, or of programmable logic networks.
The antifuses that are concerned here are antifuses that can be programmed electrically. They have the advantage of enabling the modification of the integrated circuit after the complete encapsulation of the circuit, and even after insertion into a chip card. A programming circuit is therefore associated with the antifuse to enable the antifuse to be programmed with a command sent from outside the integrated circuit.
The term "fuse" includes elements that are conductive in the intact state and go into an open circuit condition in the blown state or, conversely, antifuses that are insulating in the intact state and become conductive in the programmed state. The latter antifuses are more particularly but not exclusively concerned by the present invention.
2. Description of the Prior Art
In the patent application No. EP-A-0 408 419, there is a description, for example, of an antifuse that is open in the intact state, made out of a polycrystalline silicon conductor placed on top of a conductor made out of doped monocrystalline silicon, the two conductors being separated by an insulating layer that is locally very thin (about 100 angstroms thick). The very thin insulating layer is sufficient to ensure insulation between the conductors at the voltages that are normally applied to the integrated circuit. However, it can be programmed by the application of a voltage of about twenty volts. This voltage generates an electrical field, through the oxide, of several millions or tens of millions of volts per centimeter, which is greater than the breakdown threshold of the insulating material. A low value resistance is then set up between the conductors. The antifuses goes into its final state which is the programmed state. A transistor-based circuit enables the detection of the current leakages through the low-resistance connection and then makes it possible to prohibit certain functions of the integrated circuit (for example writing or reading in certain memory zones). In one application to an electrically programmable network, the programming of the antifuses sets up the connections needed to obtain the desired logic functions.
A major problem of the antifuses is related to the reliability of the antifuse in the programmed state: firstly, it is necessary to create programming conditions that will definitely (and not just probably) result in programming. Secondly, there must be certainty that a programmed fuse cannot return, under certain conditions, to its intact state (with almost infinite connection resistance) or to an uncertain state (excessively high connection resistance).
A programming voltage Vpp of about twenty volts may be applied from outside the integrated circuit, when the antifuse has to be blown. However, in order to reduce the number of connection terminals and in order to simplify the system for the user, it is preferred to design the system so that the programming voltage Vpp is generated in the very interior of the integrated circuit, using the lower voltage Vcc (generally 5 volts, but less in the future) which is used for the normal working of the circuit. This programming voltage is furthermore necessary in the non-volatile memory circuit (EEPROM), and it is therefore reasonable to seek to use the same voltage to program the fuses electrically.
Experience has shown, however, that the reliability of the programming of the fuses is not always very high, especially when the programming voltage is produced from within an integrated circuit supplied with a voltage Vcc that is smaller than the necessary programming voltage.
One of the reasons that have been found for this defect seems to be the fact that the voltage Vpp produced internally, generally with what is called a voltage multiplier or "charge pump", is not accompanied by sufficient power. In other words, the charge pump produces sufficient voltage but generally does not produce sufficient current, or in any case does not produce it in a sufficiently lasting manner.
It would seem indeed that the insufficiency of current during the programming (programming due first of all to the voltage Vpp) leads to an excessively resistive link through the thin oxide between the two conductors.
It is thought necessary obtain sufficient current (of some milliamperes) for some tens of milliseconds after the voltage-caused programming to stabilize the resistance of the connection that is set up at a sufficiently low value.